I have several builds similar to yours, all I want are slick-side uppers these days. After seeing the originals (armalite, colt) were slick sided, and the fact that I have never used one on mine confirms it. I do like lowers that have the integrated (oversize) trigger guard. I've never swapped one out, so don't see the reason for that either.
I'm definitely a fan of ambi safety switches- two reasons (and I'm right handed, that's not one). I've found the going safe to fire is easy with the thumb but going fire to safe is much easier on the right side of the receiver by pulling my index finger on the right side. Second reason is that it's easier for me to observe when someone is firing from the right side that they're on safe.
On the bolt release- I go standard and then get a BAD lever. I swap lowers around a lot and some of the more exotic bolt releases don't fit on other uppers, I don't think I'm a fan of ambi mag releases- seems too easy to bump and drop a mag. I haven't actually used one yet, so not sure.
Have fun! Building ARs is addicting- I ran out of reasons to give my wife as to why I needed another but since prices are reasonable I get away with it.
A hint for anyone looking for a a P&W- B King Firearms has great prices on Ballastic advantage barrels (and sometimes Faxon). They also sell muzzle brakes, and on the last barrel I bought, they pinned and welded the brake for $30 as part of my order.